1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and more specifically to a metal insulated semiconductor field effect transistor having a laterally modulated gate work function.
2. Discussion of Related Art
FIG. 1 illustrates a metal insulated insulating semiconductor field effect transistor (MISFET) 100. MISFET 100 includes a gate electrode 150 formed on a gate dielectric layer 120 which in turn is formed on a silicon substrate 102. Transistor 100 includes a pair source/drain regions formed on substrate 102 along laterally opposite sidewalls of gate electrode 150. The source/drain regions each typically include a shallow source/drain extension or tip region 140 and a relatively deep source/drain contact region 110 as shown in FIG. 1. A pair sidewall spacers 130 are formed along laterally opposite sidewalls of gate electrode 140. Sidewall spacers 130 are used to mask the source/drain tip implants from the heavy source/drain implants. The portion of the silicon substrate 102 located beneath the gate electrode and between the source/drain extension 140 defines the channel region 114 of the device. The gate electrode 150 typically slightly extends over the source/drain extension or tip regions 140 as shown in FIG. 1. Gate electrode 150 is typically formed from doped polycrystalline silicon. Alternatively, the gate electrode can be formed from a metal film. Metal gate electrodes 150 are likely to become mainstream technology in coming generations due to the elimination of poly depletion effects and subsequent improvement in gate control over the channel. The gate electrode can be formed from a single film or a composite stack of films. The gate electrode 150, however, has a constant or uniform work function across the device. That is, the work function of the gate electrode is constant from one source/drain region across the channel region to the other source/drain region.